The present invention relates to a duplicated equipment, and more particularly to a duplicated equipment used for a time division exchange or other operations to improve the reliability thereof, in which subordinate equipment is composed of a pair of subordinate devices used as an active device and a standby device, and each of the subordinate devices is changed from an active state to a standby state or from the standby state to the active state in accordance with a changeover signal from main equipment.
A signal processor such as a time division exchange is made up of a main equipment (for example, a call processor such as a speech path controller) and a subordinate equipment (for example, a terminal controller). In order to improve the reliability of the signal processor and to ensure the stable operation thereof, the subordinate equipment includes a pair of subordinate devices used as an active device and a standby device, and the standby device is operated instead of the active device immediately after the active device becomes inoperative. Further, data is transferred between the main equipment and the subordinate equipment, and an active-state/standby-state changeover signal is sent from the main equipment to the subordinate equipment.
In such duplicated equipment, an exclusive line for sending the active-state/standby-state changeover signal from the main equipment to the subordinate equipment is usually separated from a communication line (that is, a data highway) for transferring information between the main equipment and the subordinate equipment.
Further, a transmission system has been known in which a transmission line for the above changeover signal and the data highway are unified. (Refer to Japanese Patent Application JP-A-No. 59-163652). However, this transmission system has the following drawback. That is, when the automonous switching functions of signals transmitted between the main equipment and the subordinate equipment are lost as a result of a failure, it is impossible to perform a changeover operation for the subordinate equipment under the control of the main equipment. In a case where the active- state/standby-state changeover signal is sent from the main equipment to the duplicated subordinate equipment through the data highway (namely, the communication line) to reduce the number of transmission lines, if trouble occurs between the subordinate equipment and the data highway, it will become impossible to perform a changeover operation for the subordinate equipment, and thus the communication system composed of the main equipment and the subordinate equipment will not be able to operate normally, notwithstanding the main equipment and the subordinate equipment are both kept at an normal state.